While hydrogenation of materials is a rather straightforward process, its underlying behavior can be quite complex. In general, since the activity of molecular hydrogen is extremely low, it must be dissociated into its atomic parts to enable surface reactions (etching) and/or bulk in-diffusion (hydrogenation). Prior Art includes a number of techniques that are used to dissociate hydrogen include thermal processing techniques. These techniques include ranging from annealing in hydrogen forming gas and thermal cracking, which utilizes a hot filament to crack or dissociate molecular hydrogen. The cracking technique is most often applied for surface etching rather than hydrogenation. As such, it is normally processed at low pressure ˜10−4 Torr with the wafer held at a relatively high temperature to enhance its reactivity to hydrogen.
Exposure of material to hydrogen plasma has also been used to hydrogenate, as well as clean semiconductor surfaces. Plasma contains electrically charged particles in addition to neutral atoms and molecules. Energetic electrons in the plasma break up molecules and ionize the gas resulting in extremely hot plasma that is far from thermodynamic equilibrium. Substantial surface modification due to etching occurs, which generally makes this technique unsuitable for hydrogenation.
Ion implantation has also been used to hydrogenate materials. While the implanted dose is not limited by solubility limits, substantial lattice damage can occur as a result of the interaction between the energetic ions and lattice atoms. Furthermore, since energetic ions penetrate below the surface, implanted hydrogen is not available for surface modification (a benefit or a limitation depending upon the objective of the process). Therefore, there are substantial drawbacks to these processing techniques. Both cracking and plasma processes are inherently “hot” processes that are generally performed at low pressure, i.e. 10−4 to 10−2 Torr and, by design, substantially modify the surface; while ion implantation substantially damages the bulk over the ion range. Also, a highly ionized plasma, as well as ion implantation, can charge the semiconductor surface resulting in an electrostatic discharge that can destroy sensitive device structures on patterned wafers.
Alternatively, a photolytic process involving the use of ultraviolet (UV) light can be used to activate hydrogen processing of materials. It has many advantages over other prior-art technologies including greater process flexibility and control. This includes processes such as hydrogenation for electrical passivation of defects (located within the bulk or at internal interfaces of composite materials such as a heterostructure), which hereafter referred to only as “hydrogenation.” The advantages of UV-activated hydrogenation are extensive including an inherently low thermal budget and process flexibility. In addition, since it does not ionize hydrogen, charging of the surface does not occur. Therefore, UV-assisted processing is a highly flexible process that can be modified by the judicious choice of UV-lamp or filters to yield a photon bandwidth that is application specific.
Atomic hydrogen is a free radical consisting of a proton and an electron and is a powerful reducing agent capable of forming bonds at the surface or within the body of a semiconductor, especially with defects. Since most semiconductors possess open lattice structures (such as diamond, zinc blende or the wurzite), atomic hydrogen rapidly diffuses interstitially or along open-volume pathways such as dislocations. As such the size of atomic hydrogen becomes an important factor in determining its mobility. However, the charge-state of atomic hydrogen is affected by the position of the Fermi level, positive-charged hydrogen (H+) is often the preferred state due to its small size (compared to H− or H0). In general, hydrogen is an amphoteric dopant, i.e. can either provide an electron or a hole for electric conduction. At equilibrium, it acts to counter the dominant type of doping, i.e. in n-type material it is an acceptor, and in p-type it is a donor. In addition, the solubility of hydrogen in materials is generally quite low but has only been studied extensively in silicon. Reported values range from 102-5 cm−3 at 500 K at typical hydrogenation temperature. Therefore, hydrogen is mostly bound in semiconductors as a result of trapping by structural defects or impurities. Such bonding with defects often results in their deactivation.
Molecular hydrogen, H2, must undergo dissociation either in the gas phase or during surface adsorption to be reactive and in-diffuse into materials. Considering the large dissociation energy (4.52 eV), there is little doubt that the formation of atomic hydrogen by molecular dissociation is often the rate limiting step in any hydrogen-related process. However, it is well-known that hydrogen gas is transparent to UV, since it does not couple to UV-light due to its zero electric dipole moment, and therefore does not react with hydrogen gas. Rather, it is thought that UV-activates dissociation by changing the surface potential of materials. While the sticking coefficient of molecular adsorption on metal surfaces is relatively high, it is generally thought that the interaction of molecular hydrogen with the surface potential of semiconductors, e.g. silicon, is repulsive and inhibits hydrogen adsorption. Such changes in the surface potential occur during UV-irradiation simply as a result of surface charging by ejection of photoelectrons, i.e. electrons ejected from the surface of a material. The maximum energy of an electron ejected from a solid (Emax=hυ−W) is determined by the blocking potential (V) or the work function (W=eV), where e is the electronic charge, h is Planck's constant and υ is the photon frequency. Therefore, photons must have an energy, hυ, greater than W to yield photoelectrons. The work function in a semiconductor is not a constant but depends critically upon the Fermi level and temperature, as well as the state of the surface, e.g. presence of native oxides. Typically, the work function of most materials ranges from 3-7 eV, although values for insulators can be greater due to their large band gap. Thus, adsorption of hydrogen on the surface of many materials, and subsequent dissociation, can occur as a result of changes in the surface potential due to UV irradiation. While a detailed model for dissociation of molecular hydrogen does not exist, it is clear that the rate of dissociation is enhanced by UV irradiation. This enhancement clearly occurs as a result of UV-activated changes in the surface potential, as well as by direct interaction of the UV-light with the adsorbed hydrogen. (This assumes little or no surface leakage of the accumulated charge.) Therefore, the adsorption of hydrogen and its subsequent dissociation on the surface can be affected by application of an external electric field to the sample. Such field-induced changes, in addition to the field-induced drift within the sample, could then be used to affect the kinetics associated with the motion of hydrogen in- or out-diffusion.